Over 200 human tumors of various histopathologic types and anatomic sources have been evaluated using surface marker assays designed to detect and quantitate lymphoreticular cells, particularly macrophages, within the tumor mass. Few of the tumors contained significant numbers of lymphocytes or granulocytes, but a high proportion of those same tumors contained significant numbers of macrophages (0-80% of total tumor associated cells). That the numbers obtained on single cell suspensions represented an accurate picture of macrophages in intact tumors was demonstrated by immunohistologic quantitation of infiltration of tumor by IgGFc receptor positive cells. Those studies also established that the macrophages had infiltrated the entire tumor in most cases. Studies on the tumor associated cellular response were performed on primary and passaged 3-methylcholanthrene induced murine fibrosarcomas. Those studies were designed to assess the contribution of T lymphocytes, granulocytes and macrophages to the total tumor from the initial appearance throughout tumor progression. The results to date demonstrate that a) T lymphocytes completely infiltrate the tumor in very high numbers when the tumors are small but their relative contribution decreases dramatically with tumor progression, b) all tumors are completely infiltrated by large numbers of macrophages, and c) macrophage function as measured by phagocytic capacity decreases in parallel with decreases in T cells. At present, it remains unclear whether the cellular infiltrate represents a specific immune response to tumor antigens or a non-specific inflammatory response.